Normal hemostasis in man is governed by a complex series of interrelated mechanisms involving both cellular and humoral biochemical components. The biochemical pathway involves injury to intact endothelial cells, stimulation of platelets and activation of coagulation mechanisms. When a vessel is damaged and the subendothelium is exposed, platelets very rapidly adhere to some of the constituents of the vessel wall, notably collagen. Although platelets also adhere to other subendothelial constituents, only collagen has been reported to stimulate platelets to release their granule contents and recruit other platelets to the injury site.